1. Field of the Invention
The present invention relates in general to a turbocharger, and, more specifically, to a device for controlling the supercharging pressure of the turbocharger.
2. Description of the Present Art
In an internal combustion engine equipped with a turbochager, the turbocharger is usually controlled by means of an exhaust gas bypass mechanism which reduces the volume of exhaust gas flowing to the turbine by bypassing this exhaust gas. The pressure at the throttle throat of the turbine is determined by the capacity of the turbine. Accordingly, when a turbine with a small flow capacity is used, the torque is increased at low engine speed and decreased at high engine speed. When a turbine with a large flow volume is used, the torque is increased at high speed, but is reduced at medium and low engine speeds. Therefore, proposals have been put forward in the past for a variable capacity turbocharger in which it is possible to increase the torque from low engine speed to high engine speed by varying the geometry of the turbine in conformance with the operating conditions of the engine.
As a conventional variable capacity turbocharger and its control device, the device known from the Japanese Utility Model for opposition No. 53-50310 is given as an example. This proposal device is illustrated in FIGS. 1 to 4, in which a turbocharger is provided with a variable geometry device 3 comprising a movable member 4, a rod 5, and an actuator 6. The variable geometry device 3 is regulated by the degree of opening of a scroll throttle throat 2A according to the detected revolutions per minute of the engine and the position of an accelerator rack 9.
However, in the control device of this type of conventional variable capacity turbocharger, the degree of opening of the scroll throttle throat 2A is specifically determined from the engine RPM and the position of the accelerator rack 9. As a result, a change in the supercharging pressure will result, conforming to any change in the temperature of the inlet air, atmospheric pressure, and octane rating of the fuel, or a change in the intake air flow volume resulting from the individual characteristics of the engine. Therefore, the optimum supercharging pressure corresponding to the operating conditions of the engine cannot be obtained.
Accordingly, in order to obviate this type of problem, a supercharging pressure control device for a turbocharger, such as the device illustrated in FIG. 5, has been considered. This supercharging pressure control device for a turbocharger has a control unit 55 for providing revised control of the engine supercharging pressure based on the detected supercharging pressure value obtained from a supercharging pressure sensor 27 provided in an engine 24. The control unit 55 comprises a plurality of calculating means 61 and 66, a table look-up means 62, an adder 63, a plurality of setting means 64 and 69 for setting the objective supercharging pressure, a plurality of subtractors 65 and 70, and a compensating control means 68.
More particularly, in the variable geometry mechanism of the turbine, a feedforward control system which comprises the control means 61, the table look-up means 62, the adder 63, a solenoid valve 47, and the engine 24, compensates for the supercharging pressure control which has been performed with a first feed back control system comprising an objective supercharging pressure setting means 64, a subtractor 65, and a calculating means 66. On the other hand, in the exhaust gas bypass mechanism, the previously mentioned feed forward control system performs compensating control on the supercharging pressure control which has been performed, with a second feed back control system comprising a target supercharging pressure setting device 69, a subtractor 70, a compensating control means 68, and a solenoid valve 51.
However, in a supercharging pressure control device for a turbocharger, such as is described above, the supercharging pressure sensor 27, in the case where a breakdown such as a broken wire, etc. is produced, and the correct detected value cannot be outputted to a control unit 55, there is the problem that normal supercharging pressure control becomes impossible. In addition to this, in a high pressure, high load region, such as the region (C) illustrated in FIG. 6, when the actual supercharging pressure exceeds the normal value, the supercharging pressure sensor 27 breaks down. In the case where a detected value lower than the actual supercharging pressure is given to the control unit 55, the control system activates the supercharging pressure to an even higher value. In the worst case, engine damage can occur.